Wilson K-Factor&reg; Racquetball Racquet, 3 5/8", W/O Cover (K) blade (k)ontrolled power. Explosive features inside. Power strings. (K)arophite black Power string technology maximizes the length of each main string as they extend all the way down around the handle increasing the power up to an additional 15% Weight: 190 g. Headsize: 107 sq in. String Tension: 28-33 lbs. Primarily intended for ages 13+ (SGMA data full year 2008)